Alignment of the structural static components of a gas turbine engine to the centerline of its rotating assembly is critical to the performance and reliability of the engine. There have been two general ways to achieve this needed alignment.
One method is to use concentric diameters where one cylindrical face (the outer diameter or OD of the smaller part) fits into another cylindrical face (the inner diameter or ID of the larger part). This type of alignment is called a pilot. The advantage of the pilots is that they can center a part very precisely. The disadvantage is that the accuracy is dependent on the temperature and coefficient of thermal expansion for each material at build and all running conditions of the engine. Use of materials with significantly different coefficients of thermal expansion has not been possible using this alignment method because the gap between the ID and the OD is too large at start up, when the engine is cold. Thus, there is no alignment and the engine could fail.
The second method is the use of a radially instanced geometric feature, such as tabs and slots. The advantage of tabs and slots is that they can be employed under a wide range of temperatures and load conditions. The disadvantage is that this method is not as precise as the use of pilots due to manufacturing limitations. Especially with the use of materials with significantly different coefficients of thermal expansion, at operating temperatures, vibration and wear would cause the tabs to eventually fail.
Typically one or the other of the alignment methods is used for each component interface. The material and the temperature range of each component involved in the fit have, in the past, determined which of these two alignment methods is used. However, as noted above, neither is effective alone.